This invention relates to an improved elapsed time indicator or time temperature indicator. More particularly, it relates to time indicator or time temperature indicator having a vapor generating means and a porous carrier impregnated with a mixture of a gel-forming agent, and acid or base, and a pH sensitive indicator responsive to the movement of the vapor.
For certain applications, it has been shown that there is a need to detect the elapse of predetermined time periods using an indicator which is relatively independent of temperature change for certain temperature ranges, such as between 16.degree. C. and 27.degree. C. One such need exists for use with intravenous tubing in hospitals. Under current protocol, hospital personnel should change the intravenous tubing which is in use approximately every 24 hours. The tubing should be changed primarily due to the possibility of infection and because it tends to become clogged if used for a much longer time period. However, studies have shown that quite often this tubing is not changed every 24 hours because there is no reliable and inexpensive means to indicate that the tubing has been in use for a period of over 24 hours. Since the temperature in a hospital room is kept fairly constant, radical changes in temperature need not be considered during use. However, a time indicator device for such use should be relatively stable between about 16.degree. C. and 27.degree. C. Furthermore, it should be designed so that it may be readily and clearly attached to the tubing to be monitored. It should also be easily activated by hospital personnel.